Study indicates that transgenic microalgae are more efficient at photosynthesis

Researchers from the Los Alamos National Laboratory in the United States are developing strains of plant organisms that make better use of solar energy than wild strains.

By Elton Alisson

Agência FAPESP – Scientists point to microalgae as potential raw materials for the production of biofuels and other chemical products because they have high biomass and oil accumulation rates and grow two to ten times faster than terrestrial plants.

One of the challenges involved in making these organisms commercially viable for biodiesel production is increasing their photosynthetic efficiency, which is two to three times lower than that of terrestrial plants.

“Increasing the photosynthesis rate of microalgae for biodiesel production has become a critical issue in biofuel research,” said Angela Pedroso Tonon, a researcher in the Biosciences Division of the Los Alamos National Laboratory in the U.S., in comments to Agência FAPESP.

“By increasing the solar energy absorption capacity of these organisms, it is possible to elevate CO2 [carbon dioxide] fixation and to produce larger quantities of organic molecules such as carbohydrates. These carbohydrates can be converted into proteins, amino acids and, above all, oils,” explained the researcher, who received a FAPESP scholarship to complete her master’s degree and engage in doctoral research.

A group of researchers from the microalgae research center of the laboratory belonging to the U.S. Department of Energy (DOE) – which Tonon joined three months ago – have developed and are growing genetically engineered strains of microalgae that are capable of conducting more efficient photosynthesis than wild organisms that have not undergone genetic modification.

The results of the study were reported in an article published in the journal Algal Research and were presented by Tonon in an October 2014 lecture at the Advanced School on the Present and Future of Bioenergy at the University of Campinas (Unicamp) in Brazil.

“The results of the comparison between genetically modified and wild strains of microalgae demonstrated that the former present better photosynthetic performance and higher growth rates than the strains that were not genetically modified,” Tonon said.

Modulation of the antennas

According to the researcher, to conduct photosynthesis, plants and microalgae use light-harvesting centers called “antennae” that are composed of pigments such as chlorophyll. These pigments absorb light energy and transfer it to the photosystem to produce energetically active molecules that help in CO2 fixation and, consequently, carbohydrate production.

When exposed to large quantities of light such as in the summer, these light-harvesting apparatuses become quite saturated and do not efficiently coordinate the capture of photons and the transfer of electrons for carbon fixation.

Because of this, plants lose a significant amount of light energy captured in the form of heat or fluorescence that could be used to increase CO2 fixation and carbohydrate production, including the production of sugars, among other molecules.

Although microalgae, which have large light-capturing antennas, are very efficient in capturing photons, they are not as productive when they are together with other microalgae.

When on the surface of water, microalgae capture more solar energy than they are capable of using for carbon fixation; this excess energy is dissipated to the microalgae below the surface.

“It does the plant no good to have a large quantity of pigments if, when it absorbs light, it cannot transfer and convert all of the physical energy of the captured light into chemical energy because the photosystem becomes saturated,” Tonon noted.

To increase the photosynthetic efficiency of these organisms, thereby reducing the solar energy waste and causing the microalgae on the surface and at deeper levels to absorb all of the photons, researchers at the U.S. laboratory began to modulate the size of the light-capturing antennae of microalgae species such as Chlamydomonas reinhardtii and Chlorella sorokiniana.

In this way, the photosynthetic process can be carried out more uniformly by various microalgae cultivated in tanks, for example.

“The idea is to modulate the size of the microalgae antenna to reduce or increase the amount of pigment, depending on the season,” Tonon said.

“During the summer, when there is a high incidence of sunlight, the microalgae antenna does not require a large amount of pigment. In the winter, however, when there is less solar energy, the microalgae can capture more energy with a larger amount of pigment. Because of this difference, we could adjust the transgenic strains during the different seasons,” she explained.

Control of chlorophyll b

To modulate the antennae size, the researchers reduced the levels of chlorophyll b in the organisms.

Chlorophyll b absorbs light at a different wavelength than chlorophyll a, which is also found in microalgae and other plants. Thus, chlorophyll b absorbs and transfers more solar energy than other pigments, Tonon explained.

“By regulating the production of chlorophyll b, we can control the size of the microalgae antennae,” she added.

To regulate this, the researchers modulated the expression of the gene encoding chlorophyll a oxygenase, which is the enzyme responsible for pigment formation.

By reducing the expression of gene encoding this enzyme and consequently reducing the amount of chlorophyll b in the system, the researchers generated algae with different light absorption capacities.

The transgenic algae cultures presented photosynthetic rates two times higher than those of the wild strains, as measured by the amount of oxygen produced during photosynthesis.

In addition, the transgenic cultures experienced 30% more growth than the cultures that were not genetically modified.

“By reducing the production of chlorophyll b in the microalgae, they were able to make better use of the energy captured through photosynthesis and transfer it more efficiently, without saturating the photosystem or causing harm,” Tonon said.

“When microalgae or plants absorb large quantities of energy and are unable to efficiently distribute the electrons, this energy accumulates in the photosystem and causes various forms of damage such as photooxidation,” she explained.

Doctorate in bioenergy

The Advanced School on the Present and Future of Bioenergy was held under the scope of the FAPESP funding mechanism known as the São Paulo School of Advanced Science (SPSAS).

The event at the Unicamp Institute of Chemistry brought together researchers, faculty and undergraduate and graduate students from Brazil and other countries to discuss the current state of bioenergy research and prospects for the future.

“The idea for the event was to cover all aspects related to bioenergy and to highlight the fact that the raw material used to produce biofuels and other chemical products that can be generated from biomass is not limited to sugarcane,” said Andreas Karoly Gombert, Unicamp professor and meeting organizer.

Another objective of the event was to promote the Integrated Bioenergy Doctoral Program, which is jointly promoted by Unicamp, the University of São Paulo (USP) and São Paulo State University (UNESP), the first term of which began in March 2014.

“Because we wanted to achieve the highest possible degree of internationalization and attract students from outside of Brazil to the course, we thought the event would be a good way to publicize it,” Gombert said.

The article, Optimization of photosynthetic light energy utilization by microalgae (doi: 10.1016/j.algal.2012.07.002), by Perrine et al may be read in the journal Algal Research at: www.sciencedirect.com/science/article/pii/S2211926412000288.